The generation of hydrogen from natural gas via steam reforming is a well established commercial process. Natural gas is combined with steam in a hydrogen reactor where hydrogen and carbon dioxide are formed. One drawback is that commercial units tend to be extremely large in volume and subject to significant amounts of methane slip, identified as methane feedstock which passes through the reformer un-reacted. Excess carbon dioxide in the reactor slows down or stops the reaction, resulting in methane slip.
To reduce the size and increase conversion efficiency of the units, a process has been developed which uses calcium oxide to improve hydrogen yield by removing carbon dioxide generated in the reforming process. See U.S. patent application Ser. No. 10/271,406 entitled “HYDROGEN GENERATION APPARATUS AND METHOD”, filed Oct. 15, 2002, commonly owned by the assignee of the present invention, the disclosure of which is incorporated herein by reference. The calcium oxide reacts with the CO2 in a separation reaction, producing a solid calcium carbonate (CaCO3) and absorbing the CO2.
To regenerate a solid form of calcium oxide (CaO) for continued reaction with the CO2, the solid CaCO3 particles can then be placed in a calciner wherein they are heated according to the following reaction: CaCO3+heat→CaO(s)+CO2 (g). This reaction drives off the carbon dioxide (CO2) gas leaving solid CaO particles. The solid CaO is then returned for reuse in the reforming process. Rotary kiln calcination units are known which can regenerate the solid CaO, but these units are detrimentally limited by their size, high operating temperature (commonly above 1523° C. (2800° F.)), and high residence time (greater than 3 seconds) which can result in scintering of the calcium particles and subsequently reduced capability of the regenerated CaO to react CO2. A further limitation on known calcium carbonate reuse processes is that known rotary kiln type calciner units produce appreciable quantities of nitrous oxide (NO1, NO2 or NO3, hereinafter referred to as NOx) pollutant because of their high operating temperatures (commonly above 1523° C. (2800° F.)).